Helmholtz resonators are added to loudspeakers for three main reasons:
(i) to provide extension at low frequency by tuning at or near a bottom end of an operating band associated with a loudspeaker driver;
(ii) to provide acoustic filtering by tuning at or near a top end of the operating band; and
(iii) to create cone minima in the operating or pass band.
In each case the physical form of the resonator is easily recognizable as a chamber containing a volume of air and a vent duct. The present invention may make use of a Helmholtz resonator for an entirely different reason and in a form that may be distinctly different to Helmholtz resonators of the prior art.
The present invention is suited to a loudspeaker system having infinite baffle topology. Although the term “infinite” is used to describe baffles they are not literally infinite, but rather are very large in effect. For example the walls, ceiling or floor of a room, or the roof, walls or floor of a vehicle may be regarded as infinite baffles for practical purposes.
One potential problem associated with application of infinite baffle topology to loudspeakers in vehicles is structural weakening. For example cutting large holes, such as for a 12 inch loudspeaker driver in any part of a vehicle may cause structural weakening.
One known way around this problem is to mount the loudspeaker driver in a separate box and to channel sound to a listening environment through a much smaller opening.
There are several known ways of doing this. One way is via suitably designed waveguides. Another way is to use a vent duct associated with a Helmholtz resonator to penetrate a rear parcel shelf or deck of a vehicle to channel the sound to the listening environment.
Known Helmholtz resonators used to penetrate parcel shelves in vehicles are tuned in traditional ways to create band pass alignments and/or to extend low frequency response and/or to create cone minima in the pass band as described above. When used in these ways prior art infinite baffle topology loudspeaker systems using Helmholtz resonators are inherently large. Infinite baffle topology loudspeakers without Helmholtz resonators roll off at a low end of their operating band with a similar cut off frequency to sealed box topology loudspeakers. This arrangement cannot provide low frequency extension.
The present invention may provide a loudspeaker assembly comprising an electro-acoustic transducer or driver and at least one Helmholtz resonator suitable for use in a loudspeaker system. The loudspeaker assembly may be relatively small in size and may have a relatively high sensitivity. It may also have a relatively very low cut off frequency compared to sealed box topology for a same or similar driver.
Prior art teaches that low frequency extension is achieved by tuning low, near the desired low frequency cut off. It is counter intuitive in prior art that low frequency extension could be achieved by tuning higher, above the operating band of the loudspeaker assembly.
Tuning higher to provide low frequency extension would have an advantage that the loudspeaker assembly may be very small. The loudspeaker may be made as small as desired to satisfy practical requirements including cost and space availability. In some applications it may be appropriate to make the loudspeaker assembly even smaller to achieve a desired response.
Reactive components of a loudspeaker system comprising a driver mounted in a baffle may be modelled as a parallel resonant circuit. Reactive components of a Helmholtz resonator may be modelled as a series resonant circuit. When a Helmholtz resonator is added to a loudspeaker driver mounted in a baffle the components of the series resonant circuit interact with the components of the parallel resonant circuit to produce:                a) a lower tuned frequency which determines a low frequency roll off commonly called the “low cut off frequency”;        b) an intermediate tuned frequency commonly called the “box tuning” or “port tuning”; and        c) an upper tuned frequency which determines a high frequency roll off.        
For avoidance of doubt a reference to a frequency well above an operating band is a reference to the upper tuned frequency.
A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was, in Australia, known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.
Throughout the description and claims of this specification, the word “comprise” and variations of the word, such as “comprising” and comprises”, is not intended to exclude other additives or components or integers.